Description: The long-term goal of the proposed experiments is to identify the fundamental processing principles and strategies that underlie the cortical transformations and representations of complex sounds. We propose here to explore the functional organization of the primary auditory cortex, AI, in adult carnivores (Felis catus) with specific reference to its laminar organization. A main premise is that AI contains wide ranges of neuronal response properties in the main thalamo-cortical input layers and that neurons with similar functional properties are typically clustered in subregions of AI. Inter-relations between non-homogeneous spatial distributions of functional properties and structural organizations of the primary auditory cortex will be studied using combined electrophysiological and neuroanatomical methods. By relating the functional properties and distributions of neurons to aspects of cortical organization, general principles underlying the characteristics of cortical representations and transformations can be assessed for simple and complex sounds. The first aim is to define layer-specific functional organizations of AI for simple and complex sounds. It is hypothesized that single neurons in cortical layers III, IV, and V will show differences in their receptive field attributes for pure tones, spectral envelope features of broad-band stimuli, and for repetitive or sequential stimuli. The laminar distribution of receptive field properties for pure tone stimulation will be obtained from single units with reference to multiple unit maps. The second aim is to identify the thalamic input pattern to functional subregions in AI. It is hypothesized that the thalamic input patterns to functionally distinct subregions in AI differ. To explore the thalamocortical projections to functionally distinct portions of the isofrequency axis, red and green fluorescent latex microspheres will be injected at two different locations within the same frequency band to serve as retrograde neuronal tracers. The third aim is to relate intracortical connections to functional subregions of AI. It is hypothesized that patchy corticocortical projections are related to some aspects of the patchy functional organizations. These experiments will provide evidence whether some functional subregions with similar spectral, intensity, or temporal processing capacities show greater intracortical connectivity than subregions with distinctly different functional properties. To explore the intrinsic projection patterns within AI, small micropressure injections of biotinylated dextran amine (BDA) will be applied and the distribution of anterograde terminal label in AI will be assessed with reference to multiple unit maps.